Powder composition

ABSTRACT

The present invention concerns a metal powder composition for the powder metal industry, wherein the metal powder is selected from the group consisting of an atomised iron-based powder or a sponge iron powder, and a lubricant composition comprising glyceryl stearate.

FIELD OF THE INVENTION

[0001] The present invention relates to a metal powder composition for the powder metal industry. Particularly the invention relates to an iron-based metal powder composition comprising glyceryl stearate as lubricant.

BACKGROUND OF THE INVENTION

[0002] In industry the use of metal products manufactured by compacting and sintering iron-based powder compositions is becoming increasingly widespread. A number of different products of varying shapes and thickness are being produced. One processing technique for manufacture the products from the base powders is to charge the powder into a die cavity and compact the powder under high pressures. The obtained compact is then removed from the die cavity and sintered.

[0003] The quality requirements of the products are continuously raised, and in this context one important factor is that the manufactured products have high and consistent density. Much effort is put into research to develop such products and one field within this research concerns lubrication, which is used i.a. to avoid excessive wear on the die cavity during compaction. Lubrication is accomplished by spraying a liquid dispersion or solution of the lubricant onto the die cavity surface (external lubrication) or by blending a solid lubricant powder with the iron-based powder (internal lubrication). In some cases, both lubrication techniques are utilized.

[0004] The use of external, die wall lubricants can reduce or eliminate the need for an internal lubricant, but problems accompany external lubrication techniques. First, the film thickness within the die cavity has a tendency to vary, and the lubricant dispersion is known to drip out of the die cavity during processing. Also, aqueous dispersions are a source of rust formation on the die cavity. Another problem is that various external lubricant compositions are not necessarily sufficient to adequately lower ejection forces, especially at higher compaction pressures. Finally, as a technique, the die wall lubrication does not permit high productivity in comparison with internal lubrication.

[0005] Lubrication by means of blending a solid lubricant into the iron-based powder composition has also disadvantages. One problem is that the lubricant generally has a density of about 1-1.2 g/cm³ , as compared with the density of the iron-based powder, which is about 7-8 g/cm³. Inclusion of the less dense lubricant in the composition lowers the green density of the compacted part. Second, internal lubricants are generally not sufficiently effective for reducing the ejection pressures when manufacturing parts having part heights in excess of about 2.5-5 cm. Another problem is, when the particles of internal lubricant burn off during sintering, pore spaces can be left in the compacted part, providing a source of weakness for the part. Many presently used lubricants also have the disadvantage of requiring high energies for ejecting the green compact from the die.

[0006] Another disadvantage with presently used lubricants is that they often include zinc stearate. This is due to the fact zinc stearate imparts good flow properties to metal powder compositions including this stearate. In reducing atmospheres, the zinc oxide remaining after initial decomposition of the stearate is reduced to zinc, which readily volatilises because of its low boiling point. Unfortunately, on contacting the cooler parts of the furnace or the outside atmosphere, the zinc tends to condense or reoxidise. A consequence of reactions is that the production has to be interrupted as the furnace has to be cleaned regularly.

[0007] The problems associated with zinc stearate can be avoided by the use of completely organic materials such as waxes. The wax most widely used in powder metallurgy is ethylene-bisstearamide, EBS, (available under the name Acrawax™ C or Licowax™). This material has a high melting point (140° C.) but it burns off at relatively low temperatures and leaves no metallic residue. The most serious disadvantage is its poor flow behaviour in metal powders.

[0008] The present invention is particularly directed to iron based compositions with internal lubrication and wherein the lubrication is provided by a new lubricant composition including glyceryl stearate. Specifically the iron powder in the iron based composition is an atomised powder or a sponge iron powder.

[0009] Glyceryl stearate has been mentioned in connection with iron-based metal powders for the PM-industry in U.S. Pat. No. 5,518,639 which discloses lubricant compositions containing a solid phase lubricant, such as graphite, molybdenum disulfide, and polytetrafluoroethylene in combination with a liquid phase lubricant that is a binder for the solid phase lubricant. The binder can be chosen from various classes of compounds including polyethylene glycols, polyethylene glycol esters, partial esters of C₃₋₆ polyhydric alcohols, polyvinyl esters, and polyvinyl pyrrolidones. The binder is solubilized in an organic solvent. This lubricant composition is applied to the surface of a die cavity prior to compaction of the metal powder composition. The glyceryl stearate is thus known use as a binder in connection with external lubrication and, in contrast to the lubricant according to the present invention, it is not mixed with the iron-based powder and optional additives before the compation.

[0010] Another patent which mentions glyceryl stearate in connection with metal powders is the U.S. Pat. No. 6,187,259. In this patent glyceryl stearate is mentioned together with a wide variety of other substances as an agent for providing hydrofobicity in rare earth alloy powders for the production of granules.

[0011] Additionally, the U.S. Pat. No. 5,641,920 mentions the use of glyceryl monostearate as a plasticizer/compatibilizer in powders for injection moulding. In an article “Thermal extraction of binders and lubricants in sintering” by German in Advances in Powder Metallurgy & Particulate Materials, 1996 glyceryl monostearate is also mentioned.

OBJECTS OF THE INVENTION

[0012] An object of the invention is to provide an iron-based powder metal composition comprising a lubricant resulting in compacts with high and consistent densities.

[0013] A second object of the invention is to provide an iron-based powder metal composition comprising a lubricant resulting in compacts requiring low ejection energies.

[0014] A third object of the invention is to provide an iron-based powder metal composition having good flow and comprising a lubricant which is free of zinc.

SUMMARY OF THE INVENTION

[0015] These objects as well as other objects that will be apparent from the description below have now been obtained according to the present invention by providing a metal powder composition, wherein the metal powder is selected from the group consisting of an atomised iron-based powder or a sponge iron powder, and a lubricant comprising glyceryl stearate.

DETAILED DESCRIPTION OF THE INVENTION

[0016] As used in the description and the appended claims, the expression “iron-based powder” encompasses powders prepared by atomisation, preferably water atomisation. Alternatively, the powder may be based on sponge iron. The powders may be made up essentially of pure iron; iron powder that has been pre-alloyed with other substances improving the strength, the hardening properties, the electromagnetic properties or other desirable properties of the end products; and particles of iron mixed with particles of such alloying elements (diffusion annealed mixture or purely mechanical mixture). Examples of alloying elements are copper, molybdenum, chromium, nickel, manganese, phosphorus, carbon in the form of graphite, and tungsten, which are used either separately or in combination, e.g. in the form of compounds (Fe₃P and FeMo). Unexpectedly good results are obtained when the lubricants according to the invention are used in combination with iron-based powders having high compressibility. Generally, such powders have a low carbon content, preferably below 0.04% by weight. Such powders include e.g. Distaoly AE, Astaloy Mo and ASC 100.29, all of which are commercially available from Höganäs AB, Sweden. The particles of iron based powders will have a weight average particle size in the range of above about 10 microns. Preferred are iron or pre-alloyed iron particles having a maximum weight average particle size up to about 350 microns; more preferably the particles will have a weight average particle size in the range of about 25-150 microns, and most preferably 40-100 microns.

[0017] A key feature of the present invention is the glyceryl stearate lubricant. Three forms of glyceryl stearate exist, namely mono-, di- or tristearate. Technical grade glyceryl monostearate which includes about 20% glyceryl distearate is used in a preferred embodiment of the invention.

[0018] It is preferred that the glyceryl stearate is used in combination with at least one additional lubricant, which is preferably selected from the group consisting of non-metallic fatty acid compounds, such as ethylene bis-stearamide, stearic acid, oleic acid, and/or metal salts of fatty acids such as zinc stearate, calcium stearate and lithium stearate. According to a preferred embodiment of the invention the lubricant is made up by 5-95% by weight of the glyceryl stearate and 95−5% by weight of the additional lubricant. The presently most preferred embodiment of the lubricant is a lubricant composition comprising 5-95% by weight of the glyceryl stearate and 95−5% by weight of ethylene bisstearamide One aspect of the invention concerns such a lubricant composition per se.

[0019] The total amount of lubricant(s) in the metal powder composition may vary between 0.1 and 2.0% by weight, preferably between 0.1 and 0.8% by weight.

[0020] The lubricant composition may be used as a physical mixture, but is most preferably used as a molten and subsequently solidified and micronised powder of solid particles. The average particle size of the lubricant particles may vary, but is preferably in the range of 3-150 μm. If the particle size is too large, it becomes difficult for the lubricant to leave the pore structure of the metal-powder composition during compaction and the lubricant may then give rise to large pores after sintering, resulting in a compact showing impaired strength properties.

[0021] The glyceryl stearate lubricant or lubricant composition according to the present invention may also be present as a coating on powder particles and work as a lubricant/binder as described in the U.S. Pat. No. 5,480,469 or in the WO publication 01/17716 both of which are hereby incorporated by reference.

[0022] Apart from the iron-based powder and the lubricant according to the invention, the metal powder composition may contain one or more additives selected from the group consisting of binders, processing aids and hard phases. The binder may be added to the powder composition in accordance with the method described in U.S. Pat. No. 4,834,800 (which is hereby incorporated by reference).

[0023] The binder used in the metal-powder composition may consist of e.g. cellulose ester resins, hydroxyalkyl cellulose resins having 1-4 carbon atoms in the alkyl group, or thermoplastic phenolic resins.

[0024] The processing aids used in the metal-powder composition may consist of talc, forsterite, manganese sulphide, sulphur, molybdenum disulphide, boron nitride, tellurium, selenium, barium difluoride and calcium difluoride, which are used either separately or in combination.

[0025] The hard phases used in the metal-powder composition may consist of carbides of tungsten, vanadium, titanium, niobium, chromium, molybdenum, tantalum and zirconium, nitrides of aluminium, titanium, vanadium, molybdenum and chromium, Al₂O₃, B₄C, and various ceramic materials.

[0026] With the aid of conventional techniques, the iron-based powder and the lubricant particles are mixed to a substantially homogeneous powder composition.

EXAMPLE

[0027] The following example, which is not intended to be limiting, present certain embodiments and advantages of the present invention. Unless otherwise indicated any percentages are on a weight basis.

[0028] The mixtures listed in table 1 below were prepared: TABLE 1 Sample GMS* EBS** 1 0 100 2 25 75 3 50 50

[0029] The iron-based powder in all samples was ASC100.29 which is a water atomised, high purity iron powder available from Höganäs AB, Sweden. The total amount of lubricant was 0.8% which was admixed in the powder composition together with 0.5% graphite(C-uf4) . The dry ingredients in the different test mixtures i.e. the iron powder, the lubricant and the graphite were blended to homogenous mixtures and added to a die cavity before compaction. The compaction operation was performed with the different powder mixtures at 400, 600 and 800 MPa at ambient temperature.

[0030] The different test mixtures were tested as regards green density (GD), sintered density (SD), ejection energy and flow and the results are shown in FIGS. 1-5, wherein

[0031]FIG. 1 discloses the green and sintered densities obtained with the inventive powder composition including glyceryl stearate in comparison with the same powder including the conventionally used lubricant EBS.

[0032]FIG. 2 discloses the ejection energy as a function of the green density obtained with the inventive powder composition in comparison with the same powder including the conventionally used lubricant EBS.

[0033]FIG. 3 discloses the spring back as a function of the green density obtained with the inventive powder composition in comparison with the same powder including the conventionally used lubricant EBS.

[0034]FIG. 4 discloses the flow and apparent density of the the inventive powder composition in comparison with the same powder including the conventionally used lubricant EBS. 

1. A metal powder composition, wherein the metal powder is selected from the group consisting of an atomised iron-based powder or a sponge iron powder, and a lubricant comprising glyceryl stearate.
 2. A metal powder composition according to claim 1, wherein the glyceryl stearate is a mono-, di- or tristearate or a mixture thereof.
 3. A metal powder composition according to claim 1 or 2, wherein the metal powder composition includes 0.1-2.0% by weight of the lubricant.
 4. A metal powder composition according to any of the preceding claims, wherein the lubricant includes 5-95% by weight of the glyceryl stearate and 95−5% by weight of at least one lubricant selected from the group consisting of non-metallic fatty acid compounds and a metal salts of fatty acids.
 5. A metal powder composition according to claim 4, wherein the fatty acid compound is ethylene bisstearamide.
 6. A metal powder composition according to claim 5, wherein the lubricant is a present as a molten and subsequently micronised powder of glyceryl stearate and ethylene bisstearamide.
 7. A metal powder composition according to any of the preceding claims, wherein the iron-based powder is an essentially pure iron powder, a pre-alloyed or a diffusion alloyed powder.
 8. A metal powder composition according to claim 7, wherein the alloying element(s) for the pre-alloyed or diffusion alloyed powder are selected from the group consisting of copper, molybdenum, chromium, manganese, nickel, phosphorus and carbon in the form of graphite.
 9. A metal powder composition according to claim 7, wherein an essentially pure iron powder is admixed with an alloying element selected from the group consisting of copper, molybdenum, chromium, manganese, nickel, phosphorus and carbon in the form of graphite.
 10. A metal powder composition according to any of the preceding claims, wherein the metal powder also includes one or more additives selected from the group consisting of binders, processing aids and hard phases.
 11. A metal powder composition wherein the powder particles are surrounded by a coating including glyceryl stearate.
 12. Lubricant composition comprising 5-95% by weight of a glyceryl stearate and 95−5% by weight of at least one non-metallic fatty acid compound and/or a metal salt of a fatty acid.
 13. Lubricant composition according to claim 10 wherein the non-metallic fatty acid compound is selected from the group consisting of ethylene bisstearamide, stearic acid and oleic acid.
 14. Lubricant composition according to claim 10 wherein the metal salt of the fatty acid is selected from the group consisting of zinc stearate, calcium stearate and lithium stearate.
 15. Lubricant composition according to any one of the claims 10-13 in the form of a molten and subsequently micronised powder. 